1. Field of the Invention
The present invention relates to an image processing method, and particularly refers to an image processing method suitably used for displaying change of a subject over time in medical images.
2. Description of the Related Art
As of recent, JIRA (Japan Industries Association of Radiological Systems) has set the MIPS (Medical Image Processing Systems) standards in accordance with DICOM (Digital Information and Communication in Medicine) which is an international standard regarding digitizing medical images, and accordingly, digitizing of medical images is making progress.
In the medical image field, there is increased use of CAD (Computer Aided Diagnosis) for analyzing medical X-ray images, CT scan images, MRI images, and so forth, using computers. Especially noteworthy is time-difference CAD which yields the difference over time of images taken of the same portion.
With CAD which handles the difference over time, image analysis is performed for a pair of plain orbital radiography images of the chest taken at different points in time, for example. Portions in the images which are anatomically the same are found, either the current or the past image is deformed, and difference processing is performed on each pixel.
The luminance value of the difference image obtained by such difference processing corresponds to the change in image signals in the current and past images. That is, in the event that there is no difference between the past image and current image, the difference value is 0, but in the event that there is some sort of change, there is a change in the luminance level corresponding to the change thereof.
FIGS. 22A through 22D illustrate an example of change in images signals for images taken of the same portion at different points in time. Signals of the past image and signals of the current image, and signals of the difference image obtained from these, are shown one-dimensionally.
FIG. 22A shows past image signals with a generally smooth profile. On the other hand, the region A in FIG. 22B which is a current image shows signals indicating a shadow which has appeared since. Subtracting the signals of the current image from the past image and generating difference image signals of the difference yields the profile shown in FIG. 22C.
FIG. 22D is an example of a difference image of a chest X-ray image wherein such a change has occurred, with the region A having a low luminance corresponding to the newly-appeared shadow being shown. Displaying a difference image in this way allows changes which have occurred between the two images to be observed more readily, which is advantageous, since change over time can be more readily observed from the two images.
However, it would be entirely mistaken to assume that shadows only increase or disappear from one image to another, and there are cases wherein one shadow will have increased while another has disappeared. That is to say, both appearance (or growth) and disappearance (or shrinking) of shadows may be consecutively ongoing processes, so with the above-described technique, there is the problem that the interpreter of the radiograms, i.e., the physician, must carefully observe any change in any of the images displayed in the difference image.
Also, there is the problem that in the event that there is a great number of shadows displayed in the difference image, change in the shadows is more readily overlooked, and moreover, with a great number of shadows, judging the overall progress becomes difficult.
With radiograms, there are cases wherein whether high pixel values are displayed corresponding to high luminance or corresponding to low luminance, depending on the image generating apparatus. For example, with apparatuses wherein digital images are generated by directly or indirectly converting X-rays which have passed through the subject into electric signals, as with FPDs (Flat Panel Detectors) which have come into practical use in recent years, image data is generated as a positive image wherein the pixel values of regions with high X-ray transmissivity such as in the lung field are high, and the high pixel values are displayed corresponding to high luminance on the display. Thus, the values of the corresponding pixels in the lung field are great for chest X-rays, for example. On the other hand, normal radiograms are observed as negative images, so with image data wherein the X-ray film is digitized using a film scanner or the like, the values of the corresponding pixels in the lung field are small. Accordingly, performing difference processing with both negatives and positives together means that the way the shadow is displayed is not always the same, and judgment becomes extremely difficult.